Planar light sources are currently contemplated for several different applications, such as lamps for environmental illumination, backlights in liquid crystal displays and light sources in projection displays.
Light-emitting diodes, LEDs, may be a desirable choice of light sources in many applications, for example as the life time of LEDs are higher than the life time of incandescent bulbs, fluorescent bulbs and discharge lamps.
Further, light-emitting diodes are more power consumption efficient than incandescent bulbs and are expected to be more efficient than fluorescent tubes in a near future.
In several of these and other applications, it is often desired to achieve light of high brightness and color variability.
The brightness (B) is defined as being the amount of lumens (Φ) emitted per unit of area (A) and per unit of solid angle (Ω):
  B  =            Φ              A        ⁢                                  ⁢        Ω              .  
Conventionally, color variability is obtained by arranging a number of red, green, blue and amber LEDs in an array (rows, columns or a two-dimensional matrix) to form an array of color variable, independently addressable, pixels.
Color variable light of high brightness is typically obtained by stacking a high number of high-brightness LEDs, emitting in different parts of the spectrum, side by side in a matrix. The more LEDs being arranged on a certain area, the higher the ratio Φ/A becomes.
However, positioning LEDs that emit different colors side by side in itself is not an efficient way of obtaining light that is collimated as much as possible. Typically, LED emit light in an essentially Lambertian pattern, i.e. having an intensity proportional to the cosine of the angle from which it is viewed. Positioning LEDs of different colors side by side will again result in a Lambertian radiation pattern. Thus, the angular spread, proportional to Ω, is unchanged.
Conventionally, efficient collimation is obtained by leading un-collimated light into a funnel having reflective inner walls and which has a smaller cross section at the receiving side and a larger cross section at the output side. Thus, the collimator in general has an area larger than the area of the light source. Thus, by using conventional collimators, the light sources must be in spaced apart arrangement in order for the collimators to fit, which increases the area (A) in the formula above, leading to a decreased brightness.
Further, by arranging light sources in a spaced apart arrangement, the light mixing will be negatively affected.
US2004/0120647 A1, to Sakata et al, describes an optical element for mixing light from three adjacent light sources, such as a red, a green and a blue light-emitting diode. The optical element includes a first optical wave guide having a first incidence plane on which first color light is incident and an emergence plane opposed to the first incidence plane; a second optical wave guide having a second incidence plane on which second color light is incident; a third optical wave guide having a third incidence plane on which third color light is incident, the second optical wave guide and the third optical wave guide being joined to the first optical wave guide; a first dichroic filter formed on a joint plane between the first optical wave guide and the second optical wave guide to reflect the first color light and the third color light and transmitting the second color light; and a second dichroic filter formed on a joint plane between the first optical wave guide and the third optical wave guide to reflect the first color light and the second color light and transmitting the third color light, the three colors light being emerged from the emergence plane of the first optical wave guide.
However, in such an arrangement, it is not straightforward to add a fourth light-emitting diode having a fourth color. Furthermore, there is already a clear difference in degree of collimation between different colors, even without a fourth color.